pubmed-mcp-server

Pubmed Find Related

pubmed_find_related

Read-only

Find articles related to a source article — similar content (similar), articles citing this one (cited_by), or articles this one cites (references). Uses NCBI ELink as the primary source; falls back to Europe PMC then OpenAlex when NCBI is unavailable.

Input Schema

Name	Required	Description	Default
`pmid`	Yes	Source PubMed ID
`offset`	No	Result offset for pagination (0-based). For the next page, add the number of articles returned (at most 50) to this offset.
`maxResults`	No	Maximum related articles per page, up to 50 per call. A larger value is served at 50, with a notice naming the next `offset` when more remain.
`relationship`	No	Relationship type: similar (content-based), cited_by (articles citing this one), references (articles this one cites)	similar

Output Schema

Name	Required	Description
`error`	No	Present when the call failed. Absent on success.
`notice`	No	Guidance when results are empty, a fallback provider answered, offset overshot, a fallback provider could not be reached, upstream rows were excluded for carrying no PubMed PMID, or a `maxResults` past the per-call cap of 50 was served at 50. Absent on an NCBI result page that needs none of these.
`offset`	No	Result offset used
`source`	No	Provider that answered this request
`articles`	No	Related articles
`sourcePmid`	No	Source PubMed ID
`totalCount`	No	Total related articles found before windowing. A Europe PMC or OpenAlex total may shrink to the PubMed-addressable count once a request window covers the whole upstream set, since rows without a PubMed PMID cannot be returned.
`relationship`	No	Relationship type used
`coverageFailures`	No	Reference-coverage fallbacks that failed instead of answering, so the reference set is unverified rather than confirmed absent. Absent when every provider consulted answered.

Schema Changelog

Changes observed during successful MCP inspections.

Changed4 schema fields changedNov 3, 2026

changedInput schema / properties / maxResults / description

Previous value: -"Maximum related articles"New value: +"Maximum related articles per page, up to 50 per call. A larger value is served at 50, with a notice naming the next `offset` when more remain."

changedInput schema / properties / maxResults / maximum
```
Previous value: -50New value: +500
```

changedInput schema / properties / offset / description

Previous value: -"Result offset for pagination (0-based); page through results by incrementing by maxResults"New value: +"Result offset for pagination (0-based). For the next page, add the number of articles returned (at most 50) to this offset."

changedOutput schema / properties / notice / description

Previous value: -"Guidance when results are empty, a fallback provider answered, offset overshot, a fallback provider could not be reached, or upstream rows were excluded for carrying no PubMed PMID. Absent on a clean NCBI result page."New value: +"Guidance when results are empty, a fallback provider answered, offset overshot, a fallback provider could not be reached, upstream rows were excluded for carrying no PubMed PMID, or a `maxResults` past the per-call cap of 50 was served at 50. Absent on an NCBI result page that needs none of these."

Changed1 schema field changedOct 4, 2026

changedOutput schema / properties / articles / items / properties / pubDate / description

Previous value: -"Publication date"New value: +"Publication date as YYYY-MM-DD; a month- or year-only date reads as the first day of that month or year. On an NCBI Bookshelf chapter (`docType` \"chapter\") it is the chapter's own date — its last revision, otherwise its contribution — not the year its book began."

Changed2 schema fields changedSep 23, 2026

changedOutput schema / properties / error / properties / data / properties / reason / description

Previous value: -"Machine-readable failure mode. Declared by this tool: `queue_full`: Local NCBI request queue is at capacity. `ncbi_unreachable`: NCBI E-utilities is unreachable after all retry attempts. `ncbi_deadline_exceeded`: Total request deadline expired before NCBI returned a response. `ncbi_invalid_response`: NCBI returned a body that could not be parsed (invalid XML/JSON). `ncbi_resource_not_found`: NCBI returned a structured \"not found\" error for the requested ID(s). `europepmc_unreachable`: Europe PMC was unreachable after all retry attempts. `europepmc_invalid_response`: Europe PMC returned a body that could not be parsed (invalid JSON or XML). `europepmc_invalid_input`: Europe PMC rejected the request input (empty query, unknown sort field, malformed parameter). `openalex_unreachable`: OpenAlex was unreachable after all retry attempts. `openalex_invalid_response`: OpenAlex returned a body that could not be parsed (invalid JSON). `all_providers_failed`: Every provider eligible for the requested relationship failed; none answered. Other values are possible when a failure originates below the handler."New value: +"Machine-readable failure mode. Declared by this tool: `all_providers_failed`: Every provider eligible for the requested relationship failed; none answered. Other values are possible when a failure originates below the handler."

changedOutput schema / properties / error / properties / data / properties / reason / examples

Previous value: -[
-  "queue_full",
-  "ncbi_unreachable",
-  "ncbi_deadline_exceeded",
-  "ncbi_invalid_response",
-  "ncbi_resource_not_found",
-  "europepmc_unreachable",
-  "europepmc_invalid_response",
-  "europepmc_invalid_input",
-  "openalex_unreachable",
-  "openalex_invalid_response",
-  "all_providers_failed"
-]New value: +[
+  "all_providers_failed"
+]

Changed6 schema fields changedSep 11, 2026

changedOutput schema / properties / articles / items / properties / authors / description

Previous value: -"Author string"New value: +"Author string — the first three of the record's own authors, then \"et al.\". On an NCBI Bookshelf chapter these are the chapter's authors; the book's editors are in `editors`."

addedOutput schema / properties / articles / items / properties / bookTitle

Added value: +{
+  "description": "Title of the book an NCBI Bookshelf record belongs to. Present instead of `source` on a book record; absent on a journal article.",
+  "type": "string"
+}

addedOutput schema / properties / articles / items / properties / docType

Added value: +{
+  "description": "What PubMed classifies this record as: \"chapter\" or \"book\" for an NCBI Bookshelf record, \"citation\" for an ordinary journal article. Absent when PubMed supplies none.",
+  "type": "string"
+}

addedOutput schema / properties / articles / items / properties / editors

Added value: +{
+  "description": "Editors of the containing book, kept out of `authors` so they cannot displace the record's own authors. Absent on a journal article and on a book that credits no editors.",
+  "items": {
+    "description": "One editor, \"Surname Initials\" as ESummary renders it",
+    "type": "string"
+  },
+  "type": "array"
+}

addedOutput schema / properties / articles / items / properties / publisherName

Added value: +{
+  "description": "Publisher of the book an NCBI Bookshelf record belongs to. Present only on a book record; absent on a journal article.",
+  "type": "string"
+}

changedOutput schema / properties / articles / items / properties / source / description

Previous value: -"Journal source"New value: +"Journal the article appeared in. Absent on an NCBI Bookshelf record, which has no journal — its venue is in `bookTitle` and `publisherName` instead, and `docType` says which kind of record it is."

Changed3 schema fields changedSep 11, 2026

addedOutput schema / properties / coverageFailures

Added value: +{
+  "description": "Reference-coverage fallbacks that failed instead of answering, so the reference set is unverified rather than confirmed absent. Absent when every provider consulted answered.",
+  "items": {
+    "additionalProperties": false,
+    "description": "One coverage provider that could not be checked",
+    "properties": {
+      "provider": {
+        "description": "Reference-coverage provider that failed",
+        "enum": [
+          "europepmc",
+          "openalex"
+        ],
+        "type": "string"
+      },
+      "reason": {
+        "description": "Declared failure reason, e.g. europepmc_unreachable or provider_disabled",
+        "type": "string"
+      },
+      "retryable": {
+        "description": "Whether a retry can reach this provider",
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "provider",
+      "reason",
+      "retryable"
+    ],
+    "type": "object"
+  },
+  "type": "array"
+}

changedOutput schema / properties / notice / description

Previous value: -"Guidance when results are empty, a fallback provider answered, offset overshot, or Europe PMC rows were excluded for carrying no PubMed PMID. Absent on a clean NCBI result page."New value: +"Guidance when results are empty, a fallback provider answered, offset overshot, a fallback provider could not be reached, or upstream rows were excluded for carrying no PubMed PMID. Absent on a clean NCBI result page."

changedOutput schema / properties / totalCount / description

Previous value: -"Total related articles found before windowing. A Europe PMC total may shrink to the PubMed-addressable count once a request window covers the whole upstream set, since rows without a PubMed PMID cannot be returned."New value: +"Total related articles found before windowing. A Europe PMC or OpenAlex total may shrink to the PubMed-addressable count once a request window covers the whole upstream set, since rows without a PubMed PMID cannot be returned."

Changed4 schema fields changedSep 9, 2026

changedOutput schema / properties / error / properties / data / properties / reason / description

Previous value: -"Machine-readable failure mode. Declared by this tool: `queue_full`: Local NCBI request queue is at capacity. `ncbi_unreachable`: NCBI E-utilities is unreachable after all retry attempts. `ncbi_deadline_exceeded`: Total request deadline expired before NCBI returned a response. `ncbi_invalid_response`: NCBI returned a body that could not be parsed (invalid XML/JSON). `ncbi_resource_not_found`: NCBI returned a structured \"not found\" error for the requested ID(s). `europepmc_unreachable`: Europe PMC was unreachable after all retry attempts. `europepmc_invalid_response`: Europe PMC returned a body that could not be parsed (invalid JSON or XML). `europepmc_invalid_input`: Europe PMC rejected the request input (empty query, unknown sort field, malformed parameter). `openalex_unreachable`: OpenAlex was unreachable after all retry attempts. `openalex_invalid_response`: OpenAlex returned a body that could not be parsed (invalid JSON). Other values are possible when a failure originates below the handler."New value: +"Machine-readable failure mode. Declared by this tool: `queue_full`: Local NCBI request queue is at capacity. `ncbi_unreachable`: NCBI E-utilities is unreachable after all retry attempts. `ncbi_deadline_exceeded`: Total request deadline expired before NCBI returned a response. `ncbi_invalid_response`: NCBI returned a body that could not be parsed (invalid XML/JSON). `ncbi_resource_not_found`: NCBI returned a structured \"not found\" error for the requested ID(s). `europepmc_unreachable`: Europe PMC was unreachable after all retry attempts. `europepmc_invalid_response`: Europe PMC returned a body that could not be parsed (invalid JSON or XML). `europepmc_invalid_input`: Europe PMC rejected the request input (empty query, unknown sort field, malformed parameter). `openalex_unreachable`: OpenAlex was unreachable after all retry attempts. `openalex_invalid_response`: OpenAlex returned a body that could not be parsed (invalid JSON). `all_providers_failed`: Every provider eligible for the requested relationship failed; none answered. Other values are possible when a failure originates below the handler."

changedOutput schema / properties / error / properties / data / properties / reason / examples

Previous value: -[
-  "queue_full",
-  "ncbi_unreachable",
-  "ncbi_deadline_exceeded",
-  "ncbi_invalid_response",
-  "ncbi_resource_not_found",
-  "europepmc_unreachable",
-  "europepmc_invalid_response",
-  "europepmc_invalid_input",
-  "openalex_unreachable",
-  "openalex_invalid_response"
-]New value: +[
+  "queue_full",
+  "ncbi_unreachable",
+  "ncbi_deadline_exceeded",
+  "ncbi_invalid_response",
+  "ncbi_resource_not_found",
+  "europepmc_unreachable",
+  "europepmc_invalid_response",
+  "europepmc_invalid_input",
+  "openalex_unreachable",
+  "openalex_invalid_response",
+  "all_providers_failed"
+]

changedOutput schema / properties / notice / description

Previous value: -"Guidance when results are empty, a fallback provider answered, or offset overshot. Absent on a clean NCBI result page."New value: +"Guidance when results are empty, a fallback provider answered, offset overshot, or Europe PMC rows were excluded for carrying no PubMed PMID. Absent on a clean NCBI result page."

changedOutput schema / properties / totalCount / description

Previous value: -"Total related articles found before windowing"New value: +"Total related articles found before windowing. A Europe PMC total may shrink to the PubMed-addressable count once a request window covers the whole upstream set, since rows without a PubMed PMID cannot be returned."

Changed6 schema fields changedAug 21, 2026

changedInput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedInput schema / additionalProperties
```
Added value: +false
```

changedOutput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedOutput schema / anyOf

Added value: +[
+  {
+    "not": {
+      "required": [
+        "error"
+      ]
+    },
+    "required": [
+      "sourcePmid",
+      "relationship",
+      "offset",
+      "articles",
+      "totalCount",
+      "source"
+    ]
+  },
+  {
+    "required": [
+      "error"
+    ]
+  }
+]

addedOutput schema / properties / error

Added value: +{
+  "additionalProperties": {},
+  "description": "Present when the call failed. Absent on success.",
+  "properties": {
+    "code": {
+      "description": "JSON-RPC error code for this failure.",
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "data": {
+      "additionalProperties": {},
+      "properties": {
+        "reason": {
+          "description": "Machine-readable failure mode. Declared by this tool: `queue_full`: Local NCBI request queue is at capacity. `ncbi_unreachable`: NCBI E-utilities is unreachable after all retry attempts. `ncbi_deadline_exceeded`: Total request deadline expired before NCBI returned a response. `ncbi_invalid_response`: NCBI returned a body that could not be parsed (invalid XML/JSON). `ncbi_resource_not_found`: NCBI returned a structured \"not found\" error for the requested ID(s). `europepmc_unreachable`: Europe PMC was unreachable after all retry attempts. `europepmc_invalid_response`: Europe PMC returned a body that could not be parsed (invalid JSON or XML). `europepmc_invalid_input`: Europe PMC rejected the request input (empty query, unknown sort field, malformed parameter). `openalex_unreachable`: OpenAlex was unreachable after all retry attempts. `openalex_invalid_response`: OpenAlex returned a body that could not be parsed (invalid JSON). Other values are possible when a failure originates below the handler.",
+          "examples": [
+            "queue_full",
+            "ncbi_unreachable",
+            "ncbi_deadline_exceeded",
+            "ncbi_invalid_response",
+            "ncbi_resource_not_found",
+            "europepmc_unreachable",
+            "europepmc_invalid_response",
+            "europepmc_invalid_input",
+            "openalex_unreachable",
+            "openalex_invalid_response"
+          ],
+          "type": "string"
+        },
+        "recovery": {
+          "additionalProperties": {},
+          "description": "Actionable next step for the caller.",
+          "properties": {
+            "hint": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "hint"
+          ],
+          "type": "object"
+        },
+        "retryable": {
+          "description": "Whether retrying may succeed.",
+          "type": "boolean"
+        }
+      },
+      "type": "object"
+    },
+    "message": {
+      "description": "Human-readable description of what went wrong.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "code",
+    "message"
+  ],
+  "type": "object"
+}

removedOutput schema / required

Removed value: -[
-  "sourcePmid",
-  "relationship",
-  "offset",
-  "articles",
-  "totalCount",
-  "source"
-]

Changed3 schema fields changedJun 11, 2026

addedOutput schema / properties / totalCount

Added value: +{
+  "description": "Total related articles found before windowing",
+  "type": "number"
+}

removedOutput schema / properties / totalFound

Removed value: -{
-  "description": "Total related articles found before windowing",
-  "type": "number"
-}

changedOutput schema / required

Previous value: -[
-  "sourcePmid",
-  "relationship",
-  "offset",
-  "articles",
-  "totalFound",
-  "source"
-]New value: +[
+  "sourcePmid",
+  "relationship",
+  "offset",
+  "articles",
+  "totalCount",
+  "source"
+]

Changed6 schema fields changedJun 9, 2026

addedInput schema / properties / offset

Added value: +{
+  "default": 0,
+  "description": "Result offset for pagination (0-based); page through results by incrementing by maxResults",
+  "maximum": 9007199254740991,
+  "minimum": 0,
+  "type": "integer"
+}

changedOutput schema / properties / notice / description

Previous value: -"Optional guidance when results are empty — e.g. invalid source PMID, or references requested for a non-PMC source. Absent on successful result pages."New value: +"Guidance when results are empty, a fallback provider answered, or offset overshot. Absent on a clean NCBI result page."

addedOutput schema / properties / offset

Added value: +{
+  "description": "Result offset used",
+  "type": "number"
+}

addedOutput schema / properties / source

Added value: +{
+  "description": "Provider that answered this request",
+  "enum": [
+    "ncbi",
+    "europepmc",
+    "openalex"
+  ],
+  "type": "string"
+}

changedOutput schema / properties / totalFound / description

Previous value: -"Total related articles found before truncation"New value: +"Total related articles found before windowing"

changedOutput schema / required

Previous value: -[
-  "sourcePmid",
-  "relationship",
-  "articles",
-  "totalFound"
-]New value: +[
+  "sourcePmid",
+  "relationship",
+  "offset",
+  "articles",
+  "totalFound",
+  "source"
+]

Changed1 schema field changedMay 16, 2026

removedOutput schema / properties / articles / items / properties / score

Removed value: -{
-  "description": "NCBI relevance score (arbitrary scale, higher = more related; only present for similar)",
-  "type": "number"
-}

Changed1 schema field changedMay 10, 2026

changedOutput schema / properties / notice / description

Previous value: -"Optional guidance when results are empty due to a known limitation — e.g. references for a non-PMC source. Absent on successful result pages."New value: +"Optional guidance when results are empty — e.g. invalid source PMID, or references requested for a non-PMC source. Absent on successful result pages."

Changed1 schema field changedApr 25, 2026

addedOutput schema / properties / notice

Added value: +{
+  "description": "Optional guidance when results are empty due to a known limitation — e.g. references for a non-PMC source. Absent on successful result pages.",
+  "type": "string"
+}

Changed1 schema field changedApr 24, 2026
- addedOutput schema / properties / articles / items / description
```
Added value: +"Related article with enriched summary"
```
Changed2 schema fields changedMar 25, 2026
- addedOutput schema / properties / articles / items / properties / pubDate
```
Added value: +{
+  "description": "Publication date",
+  "type": "string"
+}
```
- addedOutput schema / properties / articles / items / properties / source
```
Added value: +{
+  "description": "Journal source",
+  "type": "string"
+}
```
First observedMar 23, 2026

TDQS

A3.9/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds real value by disclosing the source fallback chain (NCBI ELink → Europe PMC → OpenAlex), which tells the agent results may come from different backends. It does not, however, describe how to detect which source was used or how result quality differs across them.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two sentences, front-loaded with the purpose and followed by the sourcing detail. Dense but no filler; only minor redundancy with the schema's enum text.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With an output schema present and full parameter documentation, the description supplies the remaining context an agent needs: what the tool returns conceptually and that multiple backends may serve it. A note on whether fallback results differ in fields or recency would make it complete.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and every parameter, including the relationship enum and pagination semantics, is documented in the schema itself. The description mostly restates the relationship meanings already present in the enum description, adding no syntax or edge-case detail beyond it.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (find articles related to a source article) and enumerates the three relationships it supports. This is clearly distinct from pubmed_search_articles or pubmed_fetch_articles, which do not traverse citation/content relationships.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The three relationship modes are explained inline so the agent knows which mode to pick for citation-graph versus content-similarity needs. It does not explicitly say when to prefer this over siblings like pubmed_search_articles, which keeps it from a 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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Pubmed Find Related

Input Schema

Output Schema

Schema Changelog

TDQS

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